Pleochroic dyes are roughly classified into two groups. One group dyes are those in which the direction of transition moment of visible light absorption is almost in parallel with the direction of the longer axis of the molecule and which, when dissolved as a guest molecule in a host liquid crystal, are aligned so that the direction of the longer axis of the dye molecule becomes the same as that of the axis of oriented liquid crystal molecule. Such dyes are called pleochroic dyes having parallel dichroism (or P-type dyes). The other group dyes are those in which the direction of transition moment of visible light absorption is almost perpendicular to the direction of the longer axis of molecule and which, when dissolved as a guest molecule in a liquid crystal, are aligned so that the direction of the longer axis of the dye molecule becomes the same as that of the axis of oriented liquid crystal molecule. Such dyes are called pleochroic dyes having perpendicular dichroism (or N-type dyes). The present invention relates to a liquid crystal composition containing a dye belonging to the first group, i.e., pleochroic dye having parallel dichroism.
Pleochroic dyes are characterized in that the degree of light absorption depends upon the relative relation between the direction of absorption transition moment of dye molecule and that of electric vector of light. That is, the absorption becomes maximum when the direction of absorption transition moment is in parallel with the electric vector of light, and becomes minimum when the direction of absorption transition moment is perpendicular to the electric vector of light.
Therefore, in the case of the pleochroic dyes showing parallel dichroism, as shown in FIG. 1 the absorption strength is large when the dye molecule is aligned to the direction as shown 3 with respect to the light 2 polarized in the direction shown by the arrow 1 and the absorption strength is small when aligned in the direction as shown 4 and 5.
When a nematic, cholesteric or smectic liquid crystal containing such pleochroic dye is sandwiched between facing two electrodes and a voltage is applied to it across the electrodes, the liquid crystal molecules exhibit turbulent motion or are uniformly aligned along the direction of electric field depending upon the dielectric characteristics or fluid characteristics of the liquid crystal. In this occasion, the pleochroic dye molecules also move together with the liquid crystal molecules, and hence relative relation between the direction of absorption transition moment of the pleochroic dye molecule and the electric vector of incident light is changed, and consequently, the light absorption characteristics of a liquid crystal display devices are changed.
Such a phenomenon is widely known as "guest-host effect" and a color display device using electric control can be constituted by utilizing this effect (see "Guest-Host Interaction in Nematic Liquid Crystals: A New Electro-Optic Effects" reported by G. H. Heilmeier and L. A. Zanoni in Applied Physics Letters, Vol. 13, p. 91 (1968)).
For example, when a nematic liquid crystal containing a pleochroic dye showing a parallel dichroism is interposed between two transparent electrode plates having been subjected to homogeneous orientation treatment and facing the parallel with respect to each other, the liquid crystal molecules form a homogeneous alignment wherein the long axes of the molecules are aligned in a direction parallel with the electrode planes (see FIG. 2). In this situation, the molecules 10 of pleochroic dye dissolved in the liquid crystal are also aligned so that their long axes are also parallel with the electrode planes in the same direction. When white light 11 having travelling in a direction perpendicular to the electrode plane and which is polarized in the same direction as in the alignment direction of crystals by a polarizing plate 13 is transmitted through the guest-host material in the above-described alignment, the electric vector of incident white light is parallel with the long axes of the pleochroic dye molecules, and hence a specific wavelength region of the incident light is strongly absorbed by the pleochroic dye guest material. As a result, the guest-host material appears colored. When an electric field is applied to the liquid crystal material in the above-described alignment through the transparent electrode plate, if it has a positive dielectric anisotropy, the host liquid crystal molecules 9 and the guest pleochroic dye molecules take on a homeotropic alignment wherein the axes of the molecules are perpendicularly aligned with respect to the electrode planes, due to the positive dielectric anisotropy of the host liquid crystal (see FIG. 3). In this situation, the electric vector of the incident white polarized light 11 is perpendicular to the axes of the pleochroic dye molecules, and hence the incident light is only scarcely absorbed by the pleochroic dye molecules, and the guest-host material appears weakly colored. Accordingly, the difference in color between the colored state and the weakly colored state enables the formation of optical displays by electric driving.
The above is an embodiment utilizing a nematic liquid crystal as a host liquid crystal. Other representative system of the guest-host type crystal display is utilization of cholesteric-nematic phase separation. This is the system using a liquid crystal showing a cholesteric state as a host liquid crystal. This system has two cases: one is use of a liquid crystal showing an inherent cholesteric state and other is use of a liquid crystal in which a suitable optically active material is added to an inherent nematic liquid crystal to show a cholesteric state.
For example, when a cholesteric liquid crystal containing a pleochroic dye showing a parallel dichroism and that the dielectric anisotropy is positive is interposed between two transparent plates having been subjected to homogeneous orientation treatment and facing the parallel with respect to each other, the liquid crystal molecules form the spiral molecule alignment. Similarly, the pleochroic dye molecules dissolved in the liquid crystal also form the spiral molecule alignment. When white natural light having travelling in a direction perpendicular to the electrode plane is transmitted through the guest-host material in the above-described alignment, dye molecules show various directions and hence a specific wavelength region of all polarizing components is specifically strongly absorbed by the pleochroic dye molecules. As a result, the guest-host material appears strongly colored. When an electric field is applied to the liquid crystal material in the above-described alignment through the transparent electrode plate, the spiral molecule alignment is released and the host liquid crystal molecules and the guest pleochroic dye molecules take on a homeotropic alignment wherein the long axes of the molecules are perpendicularly aligned with respect to the electric field direction. In this situation, the long axes of the pleochroic dye molecules are perpendicular to the electric vector of the incident white light, and hence the incident white light is only scarcely absorbed by the pleochroic dye molecules. As a result, the guest-host appears whitened suth that the electric field applied area is weakly colored and the display is possible.
The above is examples of a guest-host type liquid crystal display wherein the nematic liquid crystal and the cholesteric liquid crystal are used as the host liquid crystal. Even in the case of using the sumectic liquid crystal as the host liquid crystal, display is also possible by employing a suitable cell structure and driving method as similar to the nematic and cholesteric liquid crystals.
The pleochroic dyes used as a guest in a liquid crystal display utilizing the above-described guest-host effect are required to possess: (1) a high "order parameter" in a host liquid crystal; (2) a hue according to the end-use; (3) a sufficient solubility in a host liquid crystal; and (4) a high stability (light stability, heat stability, and electric stability).
Of the above-described requirements, order parameter (1) (usually presented as S) means the degree of alignment of absorption axis of dye molecule with respect to orientation direction of host liquid crystal molecules, and is defined by the following equation: ##EQU1## wherein the term of cos.sup.2 .theta. is timewise averaged, and .theta. represents an angle which the absorption axis of the dye molecule makes with the orientation direction of host liquid crystal molecules. The order parameter S of pleochroic dye molecule is experimentally determined by the following equation: ##EQU2## wherein A and A.perp. represent the absorbances of the dye molecules for the light polarized parallel to and perpendicular to the orientation direction of the host guest crystal, respectively.
Specifically, the order parameter S is a value which governs the contrast of a guest-host type liquid crystal display device. With pleochroic dyes having parallel dichroism, the nearer the value to 1 which is the theoretical maximum, the less the degree of residual color in white background parts, which serves to realize bright and highly contrasty display.
As to the hue (2) referred to hereinbefore, the dyes must satisfy requirements for a wide variety of hues, taking into consideration the purposes of coloration such as to increase displayed information, increase degree of freedom of design, and improve fashionability. Basically, if three primary colors of yellow, magenta, and cyan are obtained, all hues can be obtained by subtractive mixture of them. Therefore, with respect to the problem of hue, the three primary colors of yellow, magenta and cyan become important.
One object of the present invention is to provide a liquid crystal composition containing a pleochroic dye which satisfies the requirements (1), (3) and (4) described hereinbefore, which shows a cyan color, one of the primary colors, as the hue.
Relationship between the molecular structure of pleochroic dye and the various properties has not fully been clarified yet, and hence it is quite difficult to select a pleochroic dye which has a desired hue and satisfies all requirements described hereinbefore based on knowledges about known dyes.
With the above-described prior art in mind, the inventors have made intensive investigations and, as a result, the inventors have found anthraquinone dyes which show a cyan color and possess excellent order parameter, excellent solubility, and excellent stability, thus having achieved the present invention based on the findings.